The present invention relates to equipment of the type used in surveying and construction and, more particularly, to a detector device for such an application which detects the position or level of a reference plane defined by a rotating laser beam or defined by a stationary plane of laser light.
Laser systems have been employed in surveying and construction in which a laser beam is rotated in either a horizontal or a graded plane. U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando, illustrates a laser projector device which provides such a rotating reference beam. The rotating beam defines a plane, and various measurements can be made using the plane as a reference. For example, the elevation of a point remote from the laser beam projector device may be measured by means of a rod on which a laser beam detector is mounted. The bottom of the rod rests on the ground at the point where the measurement is to be made, and the operator moves the detector along the rod to a position where it intercepts the laser beam, as indicated by a meter or other display on the detector device. One such detector device is shown in U.S. Pat. No. 4,240,208, issued Dec. 23, 1980, to Pehrson.
A similar surveying system is shown in U.S. Pat. No. 4,732,471, issued Mar. 22, 1988, to Cain et al. In the Cain et al system, a rotating beam is not used. Rather, a laser transmitter produces an alignment field by projecting laser energy in a non-planar, stationary reference cone. When the transmitter is level, this reference cone declines from the horizontal in a amount sufficient so that some compensation is provided for positional errors which occur due to the curvature of the earth. The Cain et al patent discloses a small, hand-held device which includes a display and a photodetector module.
Both of the detector devices shown in the Cain et al and Petersen patents include a pair of adjacent, triangularly shaped photodetector elements. The orientation of the photodetector elements is such that the changes in the signal outputs from the elements which occur due to movement of the light are inversely related. As the reference light moves upward, the signal output from one of the elements increases while the signal output from the other of the elements decreases. The opposite signal changes occur when the position of the reference light moves downward. Naturally, the signal levels are also affected by the intensity of the light source and the distance of the photodetector elements from the light source. By comparing the relative signal output levels from the two photodetector elements, however, it is possible to obtain an indication of the position of the reference light without regard to the absolute intensity of the light.
While such a detector arrangement provides accurate operation under varying conditions, it has been found to be subject to error in instances when the detector device is improperly oriented. The photodetector elements are positioned behind an aperture in the case of the detector device. If the detector device is misaligned by rotation about a vertical axis such that the light does not strike the elements at normal incidence, shading of the elements by the case at the edge of the aperture can occur. If the angle of incidence is large, the photodetector elements may be entirely shaded and the device will not function. This is an inconvenience for the operator, but is simply corrected by realignment of the device. More serious, however, is the situation in which the photodetector elements are only partially shaded. In this event, the two elements are not shaded equally, thereby producing a shift in the relative signal levels from the elements, and a corresponding error in the detected position of the reference light. Since the device provides an indication of light position, the operator may be unaware of the misalignment of the device and the resulting error in its operation.
Accordingly, it is seen that there is a need for an improved device for detecting and displaying the relative position of reference light which is less sensitive to misalignment of the device.